The invention relates to a printing press designed for surface printing using a printing image carrier which has wettable and non-wettable areas in accordance with the image to be printed, there being means which under the control of a computer alter the printing image carrier for modifying the matter to be printed.
In the case of surface printing presses printing image carriers are used which are mounted on a plate cylinder and serve to transfer the image for the printing operation. For this purpose the printing image carrier has two types of area, that is to say water-repelling areas representing the inked parts of the image and water-accepting parts representing non-inked parts of the image.
During the process of printing the printing image carrier is wetted by a dampening unit with wetting fluid such as a mixture of water and alcohol so that the fluid adheres to the ink-free parts of the image on the surface of the printing image carrier. The dampening fluid is thus only displaced by the printing ink subsequently applied to the printing plate or it forms an emulsion with it. The ink applied in this manner to the printing plate is then transferred to the blanket cylinder to the sheet of paper.
Generally, the printing image is produced outside the press on the plate so that time is lost and labor required because the machine has to be stopped for changing the plate. Presently it is possible to use electronically stored data for the production of the plate which contain all the information to be printed.
The European patent No. 101,266 describes a printing press with which such electronically encoded printing information is used in order to directly produce or alter the printing plate located in the press. This means that replacement of the printing plate and the respective idle time of the machine and manual operations on the press not longer occur.
This known printing press is characterized in that the printing image carrier has a hydrophilic surface which is washed by means on the printing press, is coated with a hydrophobic layer and then acted upon by means such as a laser. The modification or re-creation of the printing image carrier only involves a short interruption of the printing operation in which the washing, coating and laser devices are operative so that the ink layer is washed off and the hydrophobic layer is renewed, which is then locally removed with the laser beam in accordance with the matter to be printed. The laser beam is modulated with the aid of the encoded print information.